A problem in the field of parenteral solution administration is that the conventional solution administration equipment is sometimes insufficiently inaccurate in its capability to administer small, precise quantities of solutions over a period of time. A chief reason for this is the problem of "cold flow" in the polyvinyl chloride solution tubing, which tends to change its dimensions under the pressure of a flow control clamp.
Attempts have been made to provide improved accuracy of parenteral solution delivery, especially at low flow rates, by means of pumped infusion systems in which the volume of solution passing to the patient is positively controlled by a pump. However, a disadvantage of this is that it is imperative for the device to be fail-safe, to avoid pumping air into an unattended patient.
In U.S. Pat. No. 4,121,584 of Turner, et al., another method and apparatus for controlling and dispensing of fluid is disclosed. Basically, an expansible volume chamber of predetermined maximum volume is equipped with an inlet and an outlet, and valves controlling flow through the inlet and outlet. This structure, when used in conjunction with parenteral solution delivery equipment for example, can provide predetermined, metered quantities of solution to the patient, while the solution administration proceeds by usual gravity head, to provide safety combined with precision volume administration.
The flow rate of solution administration is controlled by sequentially opening and closing the inlet and outlet valves. When the inlet valve is opened and the outlet valve is closed, the expansible chamber fills. Then the inlet valve is closed and the outlet valve is opened to allow the expansible chamber to drain the administration solution to the patient. The rate of administration is controlled by the timing of the opening of the upper and lower valves.
Numerous patent applications have been filed representing improvements on the invention of the Turner patent cited above, including the application of William L. Rudzena, Ser. No. 951,104 filed Oct. 13, 1978 entitled "CONTROLLED VOLUME LIQUID METER, PARTICULARLY FOR PARENTERAL SOLUTION ADMINISTRATION", now abandoned. There, the portions of the membrane which are pressed into sealing relation by the plunger with the inlet and outlet for valving action are reinforced with an additional, resilient pad member carried by the membrane wall, for reinforcing of the membrane wall. This prevents rupturing of the membrane when the plungers impinge the membrane wall against the respective inlet or outlet port where it joins the chamber, so that the membrane becomes a seal.
As an advantage of this invention, the plunger is modified so that only a predetermined amount of force can be applied by the plunger onto the membrane. Accordingly, the membrane at the inlet and the outlet can be treated in more gentle manner by the plungers, which permits the elimination of the resilient pad members, while still providing an excellent membrane life for preferably at least 24 hours continuous operation of the device of this invention, without need to replace the membrane.
Accordingly, while the reliability and life of the membrane is increased, the cost of manufacture is decreased, since there is no need for inspection to confirm the proper installation of the pads, which may be dispensed with by this invention. However, if desired, the reinforcing pads may be retained under circumstances where exceptionally long-term operation is desired.